Vacuum forming machine provided with a closed machine housing

ABSTRACT

A vacuum forming machine and a method of manufacturing a duplex twin-sheet element from two continuous webs of thermoplastic material conveyed in parallel by means of a twin-sheet frame provided therebetween, which is adapted to be loaded by means of a tensioning frame for creating closed machine chambers with the material web sections to be shaped, provides for the aspect that the twin-sheet frame is provided with a front frame section in the front along the conveying direction of said material webs, which frame section can be removed for de-moulding said twin-sheet element. The material webs are conveyed in a cycled operation when the twin-sheet frame is not loaded and open in the conveying direction, and for forming a twin-sheet element, the twin-sheet frame is closed and loaded by the tensioning frame.

The invention relates to a vacuum forming machine provided with a closed machine housing for manufacturing a duplex hollow body or twin-sheet element, respectively, constituted by two continuous thermoplastic webs of material conveyed in parallel by means of a spacer or twin-sheet frame, respectively, disposed therebetween, which is adapted for being loaded by means of a tensioning frame for creating closed machine chambers with the sections of material web to be formed, as well as to a method of manufacturing such a twin-sheet element by using such a vacuum forming machine.

Vacuum forming machines serve to shape thermoplastic material blanks or material webs, using a vacuum. A so-called twin-sheet method is known for the production of hollow bodies, wherein two material blanks or material webs are processed at the same time, with the heat of the heated thermoplastic blanks being utilised after the production of two spherical shells in order to connect these two spherical shells without additional welding or adhesive bonding in an efficient and environmentally acceptable manner. In relation to vacuum forming and welding of the two material blanks or sections of material web, which are usually tensioned in a vertically superimposed relationship, it is common to provide a spacer or twin-sheet frame between them in order to maintain the material sections or sections of the material web, respectively, at a defined distance.

The twin-sheet method according to prior art is usually realised in an open machine housing with the introduction of the material webs into so-called needle chains and their cycled transfer through one or several heating zones in succession. After the thermoplastic material webs have been heated, they are introduced into the forming station and an upper mould and a lower mould are moved together there whereupon the twin-sheet element is shaped as a hollow body. The aspect that must be considered to be a disadvantage of such multi-station machines is the fact that the thermoplastic material blanks or sections of material web are expanded when heated and more or less sag under their own weight, so that only materials with a low sagging tendency can be employed and/or small mouldings can be produced only. With such machines with an open machine housing or open-chamber machines, respectively, it is impossible, in an inexpedient way, to compensate for the sagging of the materials to be formed, which is caused by gravity.

Sagging control is possible with so-called single-station machines or with machines provided with a closed machine housing or closed-chamber machines, respectively, with these known machines, however, being configured only in the form of panel-producing machines operating on separate blanks of material.

The prevent invention is based on the problem of providing a vacuum forming machine as well as a method of manufacturing a twin-sheet element of the type defined by way of introduction, which permits material-sagging control of both material webs during the heating operation and the pre-forming operation for the continuous and low-cost manufacture of twin-sheet elements of major size.

In accordance with the present invention, this problem is solved in terms of a device by the features of claim 1 and in terms of a method by the features defined in claim 9.

Preferred device features improving the invention in an expedient manner are defined in claims 2 to 8.

The invention utilises expediently a vacuum forming machine for the manufacture of duplex mouldings or hollow mouldings or twin-sheet elements, respectively, with one closed machine housing being provided for both material webs. The twin-sheet frame must merely be relieved of its load for conveying the material webs, and after the movement of the two tool halves, merely that frame section must be removed for de-moulding of the twin-sheet part, which is front along the conveying direction of the material webs. Preferably, lateral extension takes place whereupon the twin-sheet part in the continuous strip that can be discharged out of the twin-sheet frame through the conveyor device acting up to the front frame section preferably in the form of needle chains provided in pairs and can be severed by means of a cutting means.

The twin-sheet frame is preferably configured to comprise rectangular frame sections disposed at right angles relative to each other, with all frame sections, with the exception of the front frame section along the conveying direction of the material webs, being provided in a stationary arrangement in the vacuum forming machine. The twin-sheet frame defines a cut-out in correspondence with the maximum size of the moulding, with the requirement that the shapes into which the material webs are processed on the front frame section, are mounted in flush relationship along the conveying direction of the material web.

It is expedient to provide four needle chain bar arrays, which operate in two pairs, with one pair of needle chain bar arrays conveying the upper material web whilst a second pair conveys the lower material web or material sheet, respectively. The arrangement is so made that those needle chains that are the chains conveying at the bottom and pricking into the material from the top down to the bottom at the material entry and conveying the material along the lower edge of the respective needle chain, terminate each ahead of the front frame section in order to allow for a lateral extension of the front frame section.

The realisation of the method requires that when the material webs are conveyed merely a twin-sheet frame disposed between the webs be relieved of the load, whilst that the needle chains may firmly be supported on the material webs. For the thermoforming process proper provisions are made for a downward movement of the tensioning frame, which is provided above the twin-sheet frame, thus tensioning both material webs and the twin-sheet bars for creating two closed machine chambers. The first machine chamber is formed by a stationary window plate provided in the shaping station, in relation to the lower material web, whereas the second closed machine chamber is created by the lower material web, the lower needle chain bar arrays, the closed twin-sheet frames and the upper material web.

In the following, the invention will now be described in more details, with reference to the annexed Figures wherein:

FIG. 1 is a schematic plan view of the shaping zone within the vacuum forming machine along the sectional line I-I in FIG. 2;

FIG. 2 illustrates a schematic sectional view along the sectional line II-II in FIG. 1;

FIG. 3 is a section along the sectional line III-III in FIG. 1;

FIG. 4 is a sectional view similar to FIG. 1, showing a roughly indicated material web with shaped and welded twin-sheet elements:

FIG. 5 shows a section along the sectional line V-V in FIG. 4;

FIG. 6 is a section along the sectional line VI-VI in FIG. 4;

FIG. 7 is a view similar to FIG. 4, however with a laterally extended twin-sheet frame section located in front along the conveying direction of the material webs;

FIG. 8 illustrates a section along the sectional line VIII-VIII in FIG. 7; and

FIG. 9 is a section along the sectional line IX-IX in FIG. 7.

FIGS. 1 to 3 show schematic sectional views of processing zone sections of an inventive vacuum forming machine. A lower pair of needle chain bar arrays 11 for conveying a lower tensioned material web 12 is mounted above the material web 12 only on the edge side. A tensioned upper material web 13 is arranged in parallel and spaced from the lower tensioned material web 12, with an upper pair of needle chain bar arrays 14 being provided on both sides above the upper material web 13. The needle chain bar arrays 11 and 14 prick each into the material webs 12 or 13, respectively, from the top and convey the respective material web 12 or 13 along the lower edge of the respective needle chain.

A twin-sheet frame 15 is arranged between the material webs 12 and 13, which has a rectangular configuration and consists of two solid opposite side sections 16 and 17, a rear-side frame section 18 connecting them and a front frame section 19 located in front along the conveying direction of the material webs. The frame sections 16, 17 and 18 are stationary and mounted between the material webs 12 and 13 between the material webs 12 and 13 whereas the front frame section 19 can be moved to the side for releasing the entire cross-sectional part within the twin-sheet frame 15, as will be still explained in the following. The front frame section 19 extends beyond both sides of the twin-sheet frame 15, and the needle chain bar arrays 11 and 13 terminate directly ahead of the front frame section 19.

A window plate 20 is arranged underneath the lower material web 12, and a tensioning frame 21 is located above the upper tensioned material web 13, which presents the same format as the twin-sheet frame 15. The tensioning frame 21 can be slightly raised for relieving the load for conveying the preceding material webs in the shaping station out of a position urging the upper material web 13, the twin-sheet frame 15 and the lower material web 12 against the window plate 20, whereupon the frame can be lowered for loading in order to realise the shaping operation.

FIGS. 4 to 6, which complete the FIGS. 1 to 3, show a material web with pre-shaped and welded twin-sheet elements 22 to 24 for illustrating the production of the twin-sheet elements in one continuous strip 25, with the front frame section 19 of the twin-sheet frame 15 still being in a position closing the twin-sheet frame 15. The strip 25 or the material webs 12 and 13, respectively, move in a cyclic movement along a conveying direction A, with the period of one cycle being defined by the spacing between the individual twin-sheet elements 22 to 24.

The illustrations in FIGS. 7 to 9 are distinguished from the FIGS. 4 to 6 only by the provision that the front frame section 19 is laterally moved out of the zone between the front ends 26 of the upper and lower needle chain bar pairs 11 and 14. The strip 25 with the twin-sheet elements 22 to 24 now ready for a continuing transport by the corresponding length L along the direction A for pre-forming the next twin-sheet element by vacuum. The lateral extension of the front frame section 19 is preferably carried out by pneumatic means; however it may also be carried out by means of a motor or by hydraulic means.

The inventive vacuum forming machine operates on a process control system that controls the drives of the needle chain arrays 11, 14, the loading and load-relieving operations on the tensioning frame 21, the actual vacuum forming operation, the retraction and extension of the front frame section 19, the cycled operation and the severance of shaped twin-sheet elements 23, 24. The vacuum forming operation proper is carried out in a manner known per se while in the closed machine housing the sagging of the material is controlled and possibly compensated during the heating and forming operations. The material webs are conveyed in a cycled manner while the twin-sheet frames 15 are relieved of any load and are open along the conveying direction A, as is roughly indicated in FIG. 7 for forming a twin-sheet element, the twin-sheet frame 15 is closed and loaded by the tensioning frame 21. 

1. A vacuum forming machine for manufacturing a duplex twin-sheet element constituted by two continuous webs of thermoplastic material which are conveyed in a conveying direction parallel to each other by means of a twin-sheet frame disposed therebetween, said frame being adapted to be loaded by means of a tensioning frame for creating closed machine chambers with sections of said thermoplastic material web to be formed, wherein said twin-sheet frame comprises a front frame section which is provided in front thereof, viewed in the conveying direction of said material webs and which can be removed for a de-moulding of the manufactured twin-sheet element.
 2. The vacuum forming machine according to claim 1, wherein said front frame section extends beyond the total width of said twin-sheet element to be formed.
 3. The vacuum forming machine according to claim 2, wherein said front frame section is adapted for being laterally extended.
 4. Vacuum forming machine according to claim 1, wherein said twin-sheet frame, with the exception of said front frame section, is disposed in stationary arrangement.
 5. The vacuum forming machine according to claim 1, wherein needle chain bar arrays are provided for conveying said material web, which terminate ahead of said front frame section along the conveying direction of said material webs.
 6. The vacuum forming machine according to claim 1, wherein a pair of needle chain bar arrays is provided for each material web.
 7. The vacuum forming machine according to claim 1, wherein each twin-sheet element constitutes one section of a continuous strip that can be pushed out of the vacuum forming machine after de-moulding of said twin-sheet element.
 8. The vacuum forming machine according to claim 7, wherein each twin-sheet element pushed out of the vacuum forming machine is adapted for being severed from said continuous strip by means of a cutting means.
 9. A method of manufacturing a duplex twin-sheet element from two continuous webs of thermoplastic material which are conveyed in a conveying direction parallel in a vacuum forming machine by means of a twin-sheet frame provided between said material webs, said frame being adapted to be loaded by means of a tensioning frame for creating closed machine chambers with sections of said thermoplastic material web to be formed, wherein said material webs are conveyed in a cycled operation when said twin-sheet frame is not loaded and open in said conveying direction, and wherein said twin-sheet frame is closed and loaded by said tensioning frame for forming a twin-sheet element. 